It is known that modern engines are provided with a fuel injection system for directly injecting the fuel into the cylinders of the engine. The fuel injection system generally includes a fuel common rail and a plurality of electrically controlled fuel injectors, which are individually located in a respective cylinder of the engine and which are fluid and hydraulically connected to the fuel rail through dedicated injection pipes.
Each fuel injector, particularly injectors of a common rail system, generally includes an injector housing, a nozzle and a movable needle which repeatedly opens and closes this nozzle. The fuel coming from the rail and passing through the injection pipe and, inside the injector housing, a delivery channel, reaches the nozzle and can thus be injected into the cylinder giving rise to single or multi-injection patterns at each engine cycle.
The needle movement is caused by the forces acting from above and from below the needle itself. The first one is the needle closing force, the other is the needle opening force. Both are the product between a pressure and a sealing surface. When no injection is required, the needle closing force is higher than the needle opening force, thus ensuring that the injection holes are covered. On first approximation and for a predetermined injection pressure, the needle opening force can be considered as a constant. Therefore, to cause the raising up of the needle, it will be sufficient to decrease the closing force, for example, by decreasing the pressure, which acts on top of the needle. Such pressure is due to the fuel, which fills a so called injector control volume (CV). The control volume is a small volume inside the injector housing and is delimited by injector housing walls, a first calibrated hole (known as “A” hole), a second calibrated hole (known as “Z” hole) and the top surface of the injector needle. The hole Z always joins the common rail through the injection pipe to the injector control volume. The A hole is normally closed, when no fuel injection is required, otherwise it joins the injector control volume with an injector leakage line at low pressure (as a first approximation, atmospheric pressure), when the injection is performed. The control volume is fed through the Z hole and can be emptied through the A hole. When no injection is required, being the A hole closed, the pressure in the control volume is equal to the injection pressure. When the injection is required, being the A hole larger than the Z hole, it is possible to discharge the fuel from the control volume, thus reducing the pressure in the control volume itself.
The injection is operated thanks to aid of a dedicated injector solenoid and injector actuator. The injector solenoid is controlled by an electronic control unit (ECU). The ECU operates each fuel injection, by energizing the solenoid for a predetermined period of time, causing, in turn, the injector actuator to open the A hole, the fuel discharge from the control volume to the injector leakage line, the pressure decrease in the control volume, the needle to raise up and uncover the injection holes. When the energizing time is ended, the injector actuator will close the A hole and the pressure in the control volume will increase up to the injection pressure value, causing the needle to go down and cover the injection holes. The energizing time (ET) of the fuel injector is determined by the ECU as a function of a desired quantity of fuel to be injected.
Due to functional parameters of the common rail system and the engine, mainly the pressure waves propagation in the injection pipes, the fuel injection quantity, even under same actuation conditions (rail pressure and energizing time) cannot be perfectly repeatable shot to shot. Shot to shot dispersion is physically present in each injector and determine a different injected quantity between each combustion cycle. The injector shot to shot dispersion is a core parameter that indicates how precise the injector is. In order to keep the combustion stable the shot to shot dispersion has to be minimized, ensuring the higher possible precision over temperature, engine working point and combustion mode.
Therefore a need exists for a method of operating a fuel injector which does not suffer of the above inconvenience.